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7. Pyrites

Pyrites or Iron Pyrites is a. natural ferric sulphide containing, when pure,
53-33% S and 46-67% Fe ; it is, however, usually mixed with gangue and
with other minerals, especially of copper, arsenic, cobalt, nickel and, not
infrequently, silver and gold.

Chalcopyrilc or Copper Pyrites is a double sulphide of copper and iron,
containing in the pure state 34-89% Cu, 30-54% Fe and 34-57% S ; it is
often accompanied by iron pyrites and other minerals.

The value of pyrites depends essentially on the proportions of sulphur
and copper (2 and 3, below). Other determinations usually made are
those of'moisture, arsenic and lead (i, 4 and 5).

1.  Moisture.—10 grains of coarsely powdered pyrites are kept in an
oven at 105° until of constant weight (45 hours generally suffice).

2.  Sulphur (Lunge's method).   -0-5 gram of pyrites, finely powdered
in an agate mortar and sieved through a, silk sieve, are treated with 10 c.c. of
a, mixture of 3 vols. of nitric acid (1) 1-4) and i vol. of hydrochloric acid
(free from sulphuric, acid) in a. conical furnished with a funnel, this
being heated on a water-bath so long as brown particles remain unattacked
(if any sulphur is mulcted on, a, few potassium chlorate crystals a,re added).
The liquid is then transferred to a porcelain dish -the and funnel
being well washed—and evaporated to dryness, the residue being treated
with 5 c.c. of cone. HC1 and evaporated to dryness again.    The residue is
then taken up with r. c.c. of cone. HCl and ioo c.c. of boiling water, the
solution being filtered through a small, filter and the latter well washed.1

The filtrate is neutralised with ammonia. (0 0-91) and then healed
for 15 minutes at 60-70" (not to boiling) with addition of 5 c.c. of the same
ammonia. The liquid, which should still smell distinctly of ammonia,
is filtered at once and the precipitated ferric hydroxide washed rapidly with
boiling water until, the filtrate ceases to give turbidity with barium chloride,
even after standing for some minutes. The fill rate, which should occupy
about 300 c.c,., is neutralised with pure dilute hyelroc.hloric acid (towards
methyl orange), heated to boiling with i: c.c. of cone, hydrochloric add and
precipitated with a boiling solution of barium chloride (20 c.c. of 10% solu-
tion usually suffice). After standing, the dear liquid is decanted through
a filter, the precipitate washed 3 or 4 times by decant at ion with boiling
water and transferred to the filter, washed, calcined and weighed : i part
BaSO(1 = OT3734 S.

3.  Copper (method used alt he Jhiisburg Mines)."   -5 grains ol extremely
finely powdered pyrites dried at ioo" art; treated in a conical flask with do c.c.
IIN();| (D ;•-•••••• 1-20).   When the action is at an end, the liquid is evaporated

to dryness and the residue heated until, fumes of sulphuric, acid appear, the
liquid being heated with 50 c.c. of cone,. 11(1 and 2 grams of sodium hypo-
phosphite dissolved in 5 c.c. of w:tter.   After addition of more hydro-
chloric, add and dilution with about 300 c.c. of water, Hie liquid is treated

1 The insoluble! residue, may be dried, calcined anil weighed; it; contains the silica,
silicates, barium and lead sulphates, which may occur in small .unmmts iu pyrites. If
the insoluble; residue is very small in quantity, the filtration may be omitted.                                                                                  7n aly for delicate colours, should not contain iron.                                                                       l.2o